1. Field of the Invention
The present invention relates to a vertical-type bag packaging method and apparatus in which steam-replacement deaeration is carried out. More particularly, the present invention relates to a vertical-type bag packaging method and apparatus in which a vertically suspended bag is moved, and in the course of the movement, the bag successively undergoes various processes, i.e. filling of an article to be packaged, steam-replacement deaeration to replace the air in the bag with steam, and ultrasonic sealing of the mouth of the bag.
2. Discussion of Related Art
In a so-called vertical bag-type packaging process, an article to be packaged is filled and packaged into a bag while it is moved in a vertically suspended position. It has heretofore been known to perform steam-replacement deaeration in the vertical-type bag packaging process. That is, after the article to be packaged has been filled into the bag, steam is blown into the bag to replace the air in the bag with the steam, thereby effecting deaeration. In a general vertical-type bag packaging machine having a steam-replacement deaeration device, after the steam-replacement deaeration, a seal portion of the bag mouth is pressed between a pair of hot plates heated by respective built-in heaters, thereby sealing the bag mouth.
Ultrasonic sealing is another known method of sealing the mouth of a packaging bag. In the ultrasonic sealing, the bag mouth is pressed between a horn and an anvil, and in this state, the horn is ultrasonically vibrated to generate heat in the seal portion of the bag mouth, thereby welding the seal portion. During the welding process, the filled article adhering to the inner surfaces of the seal portion can be cut off by the ultrasonic vibration and thus removed from the seal portion. Therefore, it is possible to prevent a sealing failure that would otherwise be caused by the undesired presence of the filled article bitten between the inner surfaces of the seal portion.
Japanese Patent Application Publication Nos. Hei 3-126528 and 2000-142605 disclose a technique of ultrasonically sealing a plastic tube in which hot air is blown on the inner peripheral surface of the bag mouth as a pretreatment for the ultrasonic sealing.
Japanese Patent Application Publication No. 2004-67177 discloses a vacuum packaging method and apparatus in which ultrasonic sealing is carried out after the steps of filling an article to be packaged into a bag, performing vacuum deaeration in a vacuum chamber and applying temporary sealing to the bag mouth. The temporary sealing is not a preheat treatment for the ultrasonic sealing but welding of the bag mouth to seal it. The ultrasonic seal is formed below the temporary seal to perfect the sealing of the bag mouth regardless of whether or not there is a sealing failure at the temporarily sealed position due, for example, to the undesired biting of the filled material.
During the steam-replacement deaeration, blowing steam into the bag causes a rise in temperature of the seal portion of the bag mouth. In this regard, the rise in temperature differs from place to place. In the case of using steam at 100° C., for example, the temperature in the vicinity of the widthwise center of the bag increases to from 95° C. to 100° C., for example, and the temperature in the neighborhood of the laterally opposite side edges of the bag increases to around 60° C. The above-described device for heat sealing uses hot plates having a relatively wide width in the vertical direction of the bag, e.g. a width of from 5 mm to 10 mm, to apply, directly to the seal portion, heat of a temperature at which a welding layer laminated on the inner peripheral surface of the bag melts, e.g. heat of around 160° C. in the case of using polypropylene as a welding layer material. Therefore, there is almost no effect on the seal made by the hot plates even if the temperature rise due to the steam blowing process varies according to location as stated above.
In the ultrasonic sealing process, heat is not applied to the seal portion by using an external heat source, but heat is generated in the ultrasonic seal portion as described above, and the pressing surface of the horn is narrow in width in order to efficiently transmit ultrasonic vibration to the seal portion of the bag mouth. In addition, the amount of heat generated by the ultrasonic vibration is substantially uniform independently of the position in the seal portion, that is, at any position in the longitudinal direction of the horn and the anvil. Accordingly, if the temperature rise due to the steam-replacement deaeration varies according to location, the total heat quantity varies. Consequently, incomplete welding occurs in a region where the heat quantity is insufficient. A region where the heat quantity is excessive is overheated, which may cause melting of even a layer laminated on the outside of the bag, resulting in the seal portion being melted and torn. No part of the above-mentioned patent literatures discloses the problems arising when a bag that has been subjected to steam-replacement deaeration is ultrasonically sealed, particularly how the preheat treatment should be performed, not to mention the problem of the nonuniform heat distribution over the bag caused by the steam-replacement deaeration.